Polyethylene terephthalate (PET) is a thermoplastic resin which is useful as a material for producing various molded products. Meanwhile, polyethylene naphthalate (PEN) exhibits excellent fundamental properties such as heat resistance, gas barrier property, and chemical resistance, as compared with PET. These thermoplastic polyester resins are widely used in a variety of applications including bottles, sheets and films.
However, PET or PEN exhibits high crystallinity and poses a problem in that it generates spherocrystal, which may cause turbidity or glare, during melting during the course of, for example, a molding process. Formation of a highly transparent product from PET or PEN is effectively carried out through a molding process in which crystallization is rapidly completed by performing crystallization during stretching without involving generation of spherocrystal. Thus, application of PET or PEN is limited to thin products such as bottles, films and sheets, which involve stretching when being formed; i.e., PET or PEN has not yet been widely applied to thick molded products.
As has been widely known, the crystallinity of a resin can be effectively lowered by copolymerizing or blending the resin with a third component. An amorphous polymer material can be applied to thick molded products without causing turbidity or glare, which would otherwise occur through crystallization. Therefore, in the case of production of a thermoplastic polyester resin, when the thermoplastic polyester resin is copolymerized with a monomer forming an amorphous polymer material, or when the resin is blended with an amorphous polymer material, the crystallinity of the resin can be lowered.
However, such a method poses a problem in that the glass transition temperature of the thermoplastic polyester resin is lowered through incorporation of an amorphous polymer material or a monomer thereof, and thus the heat resistance of the resin is impaired.
In general, in optical systems of various types of cameras such as a camera, a one-time-use camera and a video camera, aberration is corrected by using a plurality of concave lenses and convex lenses in combination. Specifically, chromatic aberration formed by a convex lens is corrected with chromatic aberration formed by a concave lens that is opposite that of the convex lens. In this case, the concave lens for aberration correction is required to exhibit high dispersion (i.e., low Abbe number). Optical glass or an optical transparent resin is employed as a material for an optical device used in the optical system of such a camera.
Optical glass exhibits excellent heat resistance, transparency, dimensional stability, chemical resistance and the like, and there are various optical glass materials having different refractive indexes and Abbe numbers. However, optical glass poses problems in terms of high material cost, poor moldability, and low productivity. Particularly, formation of an aspherical lens used for aberration correction requires a very sophisticated technique and high cost, which is a critical problem in practical use.
In contrast to the aforementioned optical glass, an optical transparent resin, in particular a thermoplastic transparent resin has advantages in that an optical lens can be mass-produced through injection molding of the resin, and also an aspherical lens can be readily produced from the resin. An optical lens produced from such a thermoplastic transparent resin is applied to a lens for a camera. Examples of the thermoplastic transparent resin include polycarbonate formed of bisphenol A, polymethyl methacrylate and amorphous polyolefin.
As for high dispersion (low Abbe number) of the aforementioned optical thermoplastic resins, polycarbonate formed of bisphenol A has a refractive index of about 1.59 and an Abbe number of about 32; polymethyl methacrylate has a refractive index of about 1.49 and an Abbe number of about 58; and amorphous polyolefin has a refractive index of about 1.54 and an Abbe number of about 56. Of these resins, only polycarbonate may be used as a material for producing a lens for aberration correction, but, because of its Abbe number of 32, cannot be considered to exhibit sufficiently high dispersion. Therefore, demand has arisen for a new material which can be used for producing a lens for aberration correction.
Patent Document 1 discloses a polyester resin produced through copolymerization of a fluorene dihydroxy compound, which resin has a refractive index of about 1.66 and an Abbe number of about 20, and can be used for forming a lens for aberration correction. The resin disclosed in Patent Document 1 has a low Abbe number and exhibits sufficiently high dispersion.